Conventionally, a keyboard for use in a portable personal computer or the like is so arranged that a key top descends without declining whenever the key top is depressed at any part thereof to ensure so-called smooth operation. For this reason, the conventional key switch structure has its link mechanism provided under the key top. Such a key switch having its link mechanism provided under the key top is disclosed for example in Japanese patent laid-open publication No. 2001-229764.
This key switch 200 has, as shown in FIG. 13, its link mechanism constituted of link members 202 and 204 fixed to the base part 208 of the keyboard with a holder 206. The base part 208 is constituted of a back plate 210 and a membrane sheet 212, on which the holder 206 is arranged. On the back plate 210 there is protrusively formed an embossed part 214 which is fitted in a through hole 216 penetrating the membrane sheet 212. The embossed part has a hole 218, in which there is fitted a welding pin 220 formed in the lower part of the holder 206. The back plate 210 and the holder 206 are adhered to each other having the membrane sheet 212 therebetween.
The membrane sheet is constituted of a plurality of layers 220, 222 and 224 and has a contact 226 of the key switch 200 between the layers. The layers 220, 222 and 224 cannot provide sufficient area for gluing them due to reduction in thickness and size of the keyboard, and thus, they are not sufficiently glued to each other. Consequently, in the conventional key switch structure, when the keyboard left for a long time after liquid has been spilt thereon, the liquid may enter a spot shown with an arrow 228 in FIG. 13, i.e. between the embossed part 214 and the membrane sheet 212 and further enter the contact 226 from the glued part of the membrane sheet 212.
Further, in Japanese patent laid-open publication No. 2002-216575, there is disclosed a device having a waterproofing property in preparation for the case where an operator inadvertently spills liquid on a keyboard during key switch manipulation. This keyboard device has a waterproof sheet over the membrane sheet, which covers the entire keys and is formed so that each key has a waterproof wall therearound, thereby preventing its electronics substrate and others in the lower part of the keyboard from being affected by liquid which is inadvertently spilt on the surface of the keyboard.
However, in the case of thin or small-sized keyboard, it is difficult to form a waterproof sheet in such a particular form, i.e. to form a resin film or other materials to provide a waterproof wall for each key. Moreover, a waterproof sheet thus formed is likely to be made so thick that the operator receives a repulsive force of the sheet when he or she depresses the key switch. Therefore, it takes time since the operator starts depressing the key top until the contact of the key switch is closed.